Have you referenced the state curriculum frameworks? There are specific learning standards listed for each grade, in each content area. Rather than looking at proficient or advanced standing, you could ask the question, has this student mastered the learning standards in this content area for X grade? Or is that what they were allegedly doing?
I find that I prefer end-of-course assessments, in general, in determining the appropriateness of any given placement. If she can pass the end-of-course assessment for eighth grade, then ninth grade coursework in that content area may be appropriate for her.
